Multilayer structure comprising a phase change material layer and method of producing the same

ABSTRACT

A method of producing a multilayer structure is provided, wherein the method comprises forming a phase change material layer onto a substrate, forming a protective layer, forming a further layer on the protective layer, patterning the further layer in an first 5 patterning step, patterning the protective layer and the phase change material layer by a second patterning step. In particular, the first patterning step may be an etching step using chemical etchants. Moreover, electrodes may be formed on the substrate before the phase change material layer is formed, e.g. the electrodes may be formed on one level, e.g. may forma planar structure and may not form a vertically structure.

FIELD OF THE INVENTION

The invention relates to a multilayer structure comprising a phasechange material layer, in particular to a memory cell comprising a phasechange material layer.

The invention further relates to a method of producing a multilayerstructure comprising a phase change material layer.

BACKGROUND OF THE INVENTION

In the field of non-volatile memories, flash memory scaling beyond a 45nm node has become a real issue. Technologies to face this challenge areferroelectric, magnetic and phase change memories, the latter one beingpromising for the replacement of flash and showing characteristics thatmay allow replacement of other types of memories such as DRAM. Phasechange memories are a possible solution for the unified memory being animportant step in the electronics art. OTP (“on time programmable”) andMTP (“multiple times programmable”) memories open a field that maypresent a great opportunity for phase change memories as well.

Phase change memories are based on a reversible memory switching using,for instance, chalcogenide materials. The ability of these materials toundergo fast phase transition has led to the development of rewritableoptical media (CD, DVD). The chalcogenide phase change materials may bedivided in two classes which are slightly different compositions, basedon their crystallization mechanism. The “nucleation dominated” materialGeTe-Sb₂Te₃ tie line such as Ge₂Sb₂Te₅ are generally used in ovonicunified memory (OUM) devices. In this concept, the phase change materialmay be in contact with a bottom-resistive electrode to switch reversiblyto a small volume of phase change material. “Fast growth material”,known in optical storage application (CD-RW/DVD+RW), enable very fastswitching (for instance 10 ns) with a proper phase stability.

Thus, phase change materials may be used to store information. Theoperational principle of these materials is a change of phase. In acrystalline phase, the material structure is, and thus properties are,different from the properties in the amorphous phase.

The programming of a phase change material is based on the differencebetween the resistivity of the material and its amorphous andcrystalline phase. To switch between both phases, an increase of thetemperature is required. Very high temperatures with rapid cooling downwill result in an amorphous phase, whereas a smaller increase intemperature or slower cooling down leads to a crystalline phase. Sensingthe different resistances may be done with a small current that does notcause substantial heating.

The increase in temperature may be obtained by applying a pulse to thememory cell. A high current density caused by the pulse may lead to alocal temperature increase. Depending on the duration and amplitude ofthe pulse, the resulting phase will be different. A fast cooling andlarge amplitude may quench the cell in an amorphous phase, whereas aslow cooling down and a smaller amplitude pulse may allow the materialto crystallize. Larger pulse amplitudes, so-called RESET pulses, mayamorphize the cells, whereas smaller pulse amplitudes will SET the cellto its crystalline state, these pulses are also called SET pulses.

However, the known processes for producing phase change memories may behandicapped by the fact that it is hard to pattern the phase changematerials without changing the properties of the phase change material.

OBJECT AND SUMMARY OF THE INVENTION

It may be an object of the invention to provide a multilayer structurecomprising a phase change material layer and a method of producing thesame, wherein the method may provide an efficient procedure forpatterning the phase change material layer of the multilayer structure.

In order to achieve the object defined above, a multilayer structurecomprising a phase change material layer and a method of producing thesame, according to the independent claims are provided.

According to an exemplary embodiment of the invention a method ofproducing a multilayer structure is provided, wherein the methodcomprises forming a phase change material layer onto a substrate,forming a protective layer, forming a further layer on the protectivelayer, patterning the further layer in an first patterning step,patterning the protective layer and the phase change material layer by asecond patterning step. In particular, the first patterning step may bean etching step using chemical etchants. Moreover, electrodes may beformed on the substrate before the phase change material layer isformed, e.g. the electrodes may be formed on one level, e.g. may form aplanar structure and may not form a vertically structure. Furthermore,the protective layer may comprise or may be consist of an insulatingmaterial, e.g. a nitride or an oxide dielectric.

According to an exemplary embodiment a multilayer structure comprising aphase change material is provided, wherein the multilayer structurecomprises a substrate, two electrodes, a phase change material layer,and a protective insulating layer, wherein the two electrodes arearranged on the substrate, wherein the phase change material is arrangedon the two electrodes, and wherein the insulating layer is arranged onthe phase change material layer. In particular, the insulating layer maybe arranged directly on the phase change material. Moreover, themultilayer structure may be a planar structure, e.g. the two electrodesmay be arranged on a same level and may not be arranged verticallystaggered with respect to each other. Furthermore, the phase changematerial layer may be arranged on or between the two electrodes in sucha manner that it electrically connects the two electrodes.

In this application the term “phase change material layer” mayparticularly denote any layer comprising or consisting of a materialthat has an ability to undergo fast phase transformation. In general,phase change materials (PCM) may comprise or may consist of germanium,antimony, and tellurium or mixtures thereof. In particular, chalcogenidePCM are divided in two classes with slightly different compositions,based on their crystallization mechanism. The so-called “nucleationdominated” materials along the GeTe-Sb₂Te₃ tie line such as Ge₂Sb₂Te₅and the so-called “Fast growth” materials, known in optical storageapplication (CD-RW/DVD+RW), enable very fast switching (10 ns), with animproved phase stability. They may be used in the so-called phase changeline cell concept. In this approach, the active part of the memorydevice is a PCM line formed in-between two Cu barrier electrodesdeposited on top of a Backend Of Line Process (BEOL) of a CMOS basedfront end of line.

By providing a protective insulating layer it may be possible toefficiently pattern or structure a multilayer structure comprising aphase change material layer. The protective layer may form a protectionfor the underlying phase change material layer so that even standard,aggressive etchants, like brome, fluorine, or chloride, may be usedwithout deterioration of the PCM layer. Additionally, also it may bepossible to avoid that nitrogen or oxygen plasmas will come in contactwith the PCM. Furthermore, the protective insulating layer may also forma mask for subsequent patterning steps. Moreover, the process may beimplementable or integrable into standard CMOS procedures. Inparticular, the structure and properties of the PCM may remain intact,which PCM layer may be part of an active region of a memory device.Using a method according to an exemplary embodiment of the invention mayalso avoid that the further layer, e.g. a bottom-antireflection coatinglayer (BARC), opening re-sputters the PCM at the side of a photo-resistused to pattern the further layer, leaving PCM residues after resiststrip that are extremely difficult to remove. These PCM residues couldbe removed by wet strip treatment in known procedures, which howeverwould strongly attack the PCM and/or depletes it in one or more elementsleading to altered performances of the device. Since these PCM residuesmay be avoided when using a method according to an exemplary embodimentof the invention these wet strip treatment and the resulting drawbacksmay be avoidable. Thus, it may be possible to protect the PCM frompatterning chemistries in the first patterning so that standardprocesses may be used without affecting the PCM.

A gist of an exemplary aspect of the invention may be seen in the usingof a protective layer formed on a phase change material layer whichprotective layer may be used to protect the PCM layer in patterningsteps of layers formed on the protective layer. Thus, it may be possibleto use standard patterning procedures without deteriorate the propertiesof the PCM layer. Residues of the protective layer and the patterning ofthe PCM layer may be performed by sputter dominated processes, e.g. Arsputtering, which may not deteriorate the properties of the PCM to agreat extend.

Next, further exemplary embodiments of the method of producing amultilayer structure are described. However, these embodiments alsoapply to the multilayer structure.

According to another exemplary embodiment of the method the secondpatterning step is a sputter dominated process, in particular ananisotropic sputter dominated process.

The term “sputter dominated process” may particularly denote a materialremoving process which is based mainly or predominantly on sputtering orphysical interaction, e.g. on high energetic ions. However, smallamounts of chemical etchants may be used in connection with thesputtering process. Thus, the sputter dominated process has to bedelimited against a chemical etching process in which the materialremoving is predominantly performed due to chemical interactions betweenthe material to be removed and the etchant. Such a sputter dominatedprocess may be in particular efficient for removing a phase changematerial layer since in such a sputter dominated process the phasechange material may be less altered compared to a chemical etchingprocess. In particular, the protective insulating layer may comprise amaterial adapted to protect the phase change material layer frometchants, which may be used by stripping a photoresist layer or abottom-antireflection coating.

According to another exemplary embodiment of the method in the firstpatterning step the protective insulating layer is partially patterned.In particular, the protective insulating layer may be partially removedby the first patterning step, e.g. upper or top portions of theprotective insulating layer may be removed so that a minimal protectivelayer still remains on the phase change material layer.

According to another exemplary embodiment of the method the furtherlayer is a bottom-antireflection coating.

According to another exemplary embodiment of the method a first sublayerforms the protective insulating layer and a second sublayer formed onthe first sublayer. In particular, the first sublayer may be formeddirectly onto the phase change material layer to form a first coveringlayer, while the second sublayer may be formed onto the first sublayer.Moreover, the first sublayer and the second sublayer may have differentthicknesses.

According to another exemplary embodiment of the method the firstsublayer comprises a first material, the second sublayer comprises asecond material, and the first material and the second material isdifferent. In particular, the first sublayer may consist of the firstmaterial and/or the second sublayer may consist of the second material.By providing two sublayers it may be possible to tailor the patterningprocess of the layers formed on the protective layer to the materials ofthe respective layers, while a second patterning process may be used forthe PCM layer. In particular, it may be possible that one, e.g. thefirst, of the sublayers forms a patterning or etch stop layer for apatterning step.

According to another exemplary embodiment of the method in the secondpatterning step the protective insulating layer is used as a maskinglayer for the patterning of the phase change material layer.

Summarizing, a gist of an exemplary aspect of the present invention maybe to provide a method wherein a protective layer may be formed on aphase change material (PCM) layer which may be used to protect the PCMlayer in patterning steps. This may be a new concept for patterning PCM,which may be applicable in so-called line cell PC memory. In general themethod may be applicable to any devices where the patterning of PCM isneeded. The protective layer may further serve as a hard mask during PCMpatterning and may be used to avoid contact between standard aggressiveetching chemistries and the PCM so that the properties of the PCM mayremain intact. Summarizing the control of very thin PCM layer patterningmay be improved, while standard CMOS processing may be usable which mayenable an easy integration of the method according to an exemplaryaspect of the invention into standard procedures.

The aspects and exemplary embodiments defined above and further aspectsof the invention are apparent from the example of embodiment to bedescribed hereinafter and are explained with reference to these examplesof embodiment. It should be noted that features described in connectionwith one exemplary embodiment or exemplary aspect may be combined withother exemplary embodiments and other exemplary aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail hereinafter withreference to examples of embodiment but to which the invention is notlimited.

FIG. 1 schematically illustrates a process flow for patterning a phasechange material layer according to a first exemplary embodiment of theinvention.

FIG. 2 schematically illustrates a process flow for patterning a phasechange material layer according to a second exemplary embodiment of theinvention.

FIG. 3 schematically illustrates an overview of a phase change memorybased on the so-called line cell concept.

FIG. 4 shows images showing phase change material cells.

FIG. 5 schematically illustrates work flow of a standard patterningscheme.

DESCRIPTION OF EMBODIMENTS

The illustration in the drawing is schematically. In different drawings,similar or identical elements are provided with similar or identicalreference signs.

FIG. 1 shows a schematically process flow for patterning a phase changematerial layer according to a first exemplary embodiment of theinvention. In particular, Fig. 1A shows a multilayer structure 100 whichmay be a part of a memory cell, wherein the multilayer structure 100comprises a base layer 101 and two conductor paths 102 and 103 burrowedin the substrate 101 or formed by a damascene process. The conductorpaths may be formed of metal, e.g. copper. The conductor paths 102 and103 are connected to electrodes 104 and 105, respectively, which may beformed by tantalum-nitride (TaN) for example. On the substrate 101 andthe electrodes 104 and 105 a phase change material (PCM) layer 106 isformed e.g. by using GeTe-Sb₂Te₃ such as Ge₂Sb₂Te₅. Onto the PCM layer106 a protective layer 107, e.g. a protective insulating layercomprising nitride or oxide dielectric, is formed. Afterwards aphotolithography process is used, e.g. a bottom-antireflection coating(BARC) layer 108 and a photo resist layer 109 are spin coated, exposedand the photoresist is developed.

FIG. 1B shows the multilayer structure 100 after a dry etching step isperformed to open or pattern the BARC layer 108. This step may beperformed by standard chemistry, e.g. HBr/O₂. The etching process may bestopped by endpoint detection selectively towards the protective layer107. It should be noted that the PCM layer 106 does not come in contactwith any chemistry, e.g. etchant, in that step.

FIG. 1C shows the multilayer structure 100 after the protective layer ispartially opened or patterned. In particular, the protective layer 106is partially opened by using appropriate chemistry, e.g. fluorine basedchemistry in case of a nitride or oxide dielectric. The photoresistlayer 109 serves as a masking layer for patterning the protective layer107 as well. The process is stopped before the chemistry comes incontact with the PCM layer 106.

FIG. 1D shows the multilayer structure 100 after the photoresist 109 andthe BARC layer 108 are stripped. This may be done by using standardchemistry, e.g. by using O₂, N₂ or SF₆. Since the PCM layer 106 is stillprotected by the protective layer 107 it is not deteriorated by thechemistry.

Fig. 1E shows the multilayer structure 100 after the protective layer107 is opened and the PCM layer 106 is patterned. In particular, thepatterning of the protective layer 107 is finished and the PCM layer 106is patterned by a highly anisotropic sputter dominated process. Duringthis process step the protective layer may be used as a mask to patternthe PCM layer 106.

As a consequence of this patterning scheme, the PCM layer 106 is onlyexposed to a sputter dominated process and mild chemistries during thelast etching step. All other etching steps may be performed by usingstandard chemistries and do not touch the PCM. Thus, the PCM layer 106may be reproducibly patterned without alteration of its composition.Furthermore, no residues may be created as can be seen in FIG. 4C, whichwill be described in more detail in the following.

FIG. 2 shows a schematically process flow for patterning a phase changematerial layer according to a second exemplary embodiment of theinvention. The process is similar to the process described withreference to FIG. 1. Thus, mainly the differences will be described inmore detail. The main difference is that a first sublayer 210 and asecond sublayer 211 form a protective layer 207. In particular, FIG. 2Ashows a multilayer structure 200 which may be a part of a memory cell,wherein the multilayer structure 200 comprises a base layer 201 and twoconductor paths 202 and 203 burrowed in the substrate 201 or formed by adamascene process. The conductor paths may be formed of metal, e.g.copper. The conductor paths 202 and 203 are connected to electrodes 204and 205, respectively, which may be formed by tantalum-nitride (TaN) forexample. On the substrate 201 and the electrodes 204 and 205 a phasechange material (PCM) layer 106 is formed, e.g. by using GeTe-Sb₂Te₃such as Ge₂Sb₂Te₅. Onto the PCM layer 206 a protective layer 207, e.g. aprotective insulating layer comprising nitride or oxide dielectric, isformed. As already indicated the protective layer 207 comprises tosublayers 210 and 211, e.g. a bottom layer and a top layer, which mayhave different thicknesses and may comprise or consists of differentmaterials. Afterwards a photolithography process is used, e.g. abottom-antireflection coating (BARC) layer 208 and a photo resist layer209 are spin coated, exposed and the photoresist is developed.

The patterning process is the same as described with respect to FIG. 1.However, the bottom layer 210 of the protective layer 207 may be used asa stopping layer during the patterning or opening of the top layer 211of the protective layer 207. Then the bottom layer 210 and the PCM layer206 may be etched together in the last etching step, e.g. a sputterdominated etching step.

FIG. 3 schematically illustrates a basic overview of a phase changememory based on the so-called line cell concept in which a multilayerstructure shown in FIG. 1 and FIG. 2 may be used. In particular, FIG. 3shows a PCM layer 306 connecting two electrodes 304 and 305 that areconnected to conductor paths 302 and 303, respectively. Furthermore, abase layer 301 is depicted. For sake of clarity a protective layercovering the PCM layer 306 is not shown in FIG. 3. Additionally, apassivation layer 312 is shown together with additionally layers 313 and314, e.g. made of silicone-carbide, 315 and 316, e.g. made of oxide andsome metallic layers 317 and 318.

FIG. 4 shows images a standard illustrating phase change material cells.In particular, FIG. 4A shows a bitline 401 having a first PCM region 402on top which is connected by a PCM line 403 to a second PCM region 404which is arranged on an electrode 405. A dielectric layer 406, e.g. anoxide layer, surrounds the electrode and the bitline. Furthermore, aplurality of PCM residues 407 can be seen on the PCM regions which maybe caused by an BARC opening which is stopped on the PCM layer, in caseno protective layer is used, from which the PCM regions 402 and 404 andthe PCM line is formed. FIG. 4B shows the result of a standard processin which the PCM residues shown in FIG. 4A are removed by wet stripping,leading to a strongly attacked PCM layer, e.g. PCM regions 402 and 404and PCM line 403.

In contrast to the FIGS. 4A and 4B, FIG. 4C now shows a PCM line cellmemory that is fabricated using a protective layer during patterning. Inparticular, FIG. 4C shows a bitline 411 having a first PCM region 412 ontop of it, which is connected by a PCM line 413 to a second PCM region414, which in turn is formed on an electrode 415. It can be seen in FIG.4C that the different PCM regions are less deteriorated or attacked fromthe patterning while virtually no PCM residues can be seen in FIG. 4C.

FIG. 5 schematically illustrates workflow of a standard patterningscheme without using a protective layer. In particular, FIG. 5A shows amultilayer structure 500 wherein the multilayer structure 500 comprisesa base layer 501 and two conductor paths 502 and 503 burrowed in thesubstrate 501 or may be formed by a damascene process. The conductorpaths may be formed of metal, e.g. copper. The conductor paths 502 and503 are connected to electrodes 504 and 505, respectively, which may beformed by tantalum-nitride (TaN) for example. On the substrate 501 andthe electrodes 504 and 505 a phase change material (PCM) layer 506 isformed, e.g. by using GeTe-Sb₂Te₃ such as Ge₂Sb₂Te₅. Afterwards aphotolithography process is used, e.g. a bottom-antireflection coating(BARC) layer 508 and a photo resist layer 509 are spin coated, exposedand the photoresist is developed. The standard process is similar to theone described with respect to FIG. 1 however, since no protective layeris used the PCM layer 506 will be deteriorated by the patterning steps,in particular in the step of stripping the BARC layer 508.

Finally, it should be noted that the above-mentioned embodimentsillustrate rather than limit the invention, and that those skilled inthe art will be capable of designing many alternative embodimentswithout departing from the scope of the invention as defined by theappended claims. In the claims, any reference signs placed inparentheses shall not be construed as limiting the claims. The word“comprising” and “comprises”, and the like, does not exclude thepresence of elements or steps other than those listed in any claim orthe specification as a whole. The singular reference of an element doesnot exclude the plural reference of such elements and vice-versa. In adevice claim enumerating several means, several of these means may beembodied by one and the same item. The mere fact that certain measuresare recited in mutually different dependent claims does not indicatethat a combination of these measures cannot be used to advantage.

1. A method of producing a multilayer structure, the method comprising:forming electrodes on a substrate, forming a phase change material layeronto the electrodes, forming a protective layer, forming a further layeron the protective layer, patterning the further layer in an firstpatterning step, patterning the protective layer and the phase changematerial layer by a second patterning step.
 2. The method according toclaim 1, wherein the second patterning step is a sputter dominatedprocess.
 3. The method according to claim 1, wherein in the firstpatterning step the protective layer is partially patterned.
 4. Themethod according to claim 1, wherein the further layer is abottom-antireflection coating.
 5. The method according to claim 1,wherein the protective layer is formed by a first sublayer and a secondsublayer formed on the first sublayer.
 6. The method according to claim5, wherein the first sublayer comprises a first material, wherein thesecond sublayer comprises a second material, wherein the first materialand the second material is different.
 7. The method according to claim1, wherein in the second patterning step the protective layer is used asa masking layer for the patterning of the phase change material layer.8. A multilayer structure comprising a phase change material, themultilayer structure comprising: a substrate, two electrodes, a phasechange material layer, and a protective layer, wherein the twoelectrodes are arranged on the substrate, wherein the phase changematerial is arranged on the two electrodes, and wherein the protectivelayer is arranged on the phase change material layer.